The invention relates to an ultra-flat pressure plate material tester and more particularly to ultra-flat material testers for measuring stresses.
In concrete construction, the direct measurement of absolute internal stresses is an essential element for evaluating its residual strength capacity. In the absence of any other means, a direct stress-release measuring method has been adopted. It consists of forming a kerf in the concrete with a circular saw, then re-establishing the strain-field by means of an ultra-flat material tester adapted to this kerf. The compensation pressure indicates the stress sought. This is a method used in geological survey but of which the crude transposition to concrete had, in the past, left failures. In fact, the differences in scale and in nature between the two materials require the taking into account of an increased number of parameters.
Through the documents U.S. Pat. No. 4,326,420 and DE-A No. 2,904,844, material testers are already known for measuring forces in a kerf formed in concrete or another medium.
However, testers, designed in a circular shape, have within a kerf formed by a circular saw, a surface which is always different from the surface freed by the kerf, the difference being further increased by the fact that the effective surface of the tester is reduced by reason of its convex deformation.
Through this fact, the testers cannot be used simply and reliably for measuring absolute internal stresses, and still less to achieve a stress gradient through a range of kerfs of standardized sizes.